1. Field of the Invention
The present invention relates to a rectifier circuit device used for a power source circuit or the like and a DC/DC converter provided with the rectifier circuit device, and more particularly to a rectifier circuit device and a DC/DC converter provided with which an attempt is made to reduce the size thereof.
2. Description of the Related Art
Conditions which are required of a power source circuit include improvement in efficiency of the circuit and reduction in the size thereof. Generally, the rectifier circuit device has been composed only of diodes, but a method of using a field effect transistors (FET) as an element for rectification has been developed and is known.
FIG. 1 is a block diagram showing a structure of a conventional DC/DC converter in which a rectifier circuit device is composed only of diodes. Hereinafter, this conventional DC/DC converter is referred to as a first conventional example.
In the first conventional example, a switching element 101 and a controlling circuit 100 therefor are provided on the primary side of a transformer 102. Furthermore, a rectifier circuit device 103a and a smoothing circuit 104 are provided on the secondary side of the transformer 102.
In the rectifier circuit device 103a, two diodes 105 and 106, and a first to a third external connection terminals 111 to 113 are provided. The anode of the diode 105 is connected to the first external connection terminal 111. The anode of the diode 106 is connected to the second external connection terminal 112. The cathodes of the diodes 105 and 106 are connected to the third external connection terminal 113. Then, the first external connection terminal 111 is connected to a negative pole of the transformer 102 and the smoothing circuit. The second external connection terminal 112 is connected to the positive pole of the transformer 102. The third external connection terminal 113 is connected to the smoothing circuit.
In the first conventional example thus configured, parts which incorporate two diode elements exist with the result that one part can constitute the rectifier circuit. However, the first conventional example cannot sufficiently attain improvement in the efficiency of the device and reduction in size thereof.
Then, as described above, a method has been developed, wherein an FET is used as an element for rectification. FIG. 2 is a block diagram showing a structure of a conventional DC/DC converter in which the rectifier circuit device is provided with a FET for rectification and a diode for reflux. Hereinafter, this conventional DC/DC converter is referred to as a second conventional example. Incidentally, in the second conventional example shown in FIG. 2, the same constitution elements as the first conventional example shown in FIG. 1 are denoted by the same reference numerals, and a detailed explanation thereof will be omitted.
In the second conventional example, a rectifier circuit portion 103b is connected between the secondary side of the transformer 102 and the smoothing circuit 104. In the rectifier circuit portion 103b, a diode 105 and an FET element 107 are provided. The drain of the FET element 107 is connected to the negative pole of the transformer 102. The gate of the FET element 107 is connected to the positive pole of the transformer 102 and the smoothing circuit 104. Then, the source of the FET element 107 is connected to the smoothing circuit 104. On the other hand, the cathode of the diode 105 is connected to the positive pole of the transformer 102 and the smoothing circuit 104 while the anode of the diode 105 is connected to the source of the FET element 107 and the smoothing circuit 104. Incidentally, the diode 105 and the FET element 107 are different parts.
In this manner, in accordance with the second conventional example which is constituted with the combination of the FET element and the diode e, it is possible to improve the efficiency of the power source.
However, the number of the parts which constitute the rectifier circuit portion increases, and the circuit becomes complicated. As a consequence, there arise problems in terms of the reduction in the size of the DC/DC converter and the reduction in the cost thereof.
Furthermore, in the case where the FET element is used in the rectifier circuit portion, the rectifier circuit portion is composed of two parts, one part for rectification and the other for reflux. Then, wiring is required to be provided on the printed board. Therefore, this becomes a factor which hinders attempts of the simplification of the wiring on the printed board and of improvement in the packaging efficiency thereof.
There have been disclosed a converters or the like (Japanese Patent Application Laid-Open No. Hei 6-343263, No. Sho 59-47753, No. Hei 1-144663, No. Hei 5-29908 and No. Hei 10-146047).
According to a converter disclosed in Japanese Patent Application Laid-Open No. Hei 6-343263, an FET element is provided on the secondary side of a transformer for rectification. However, as described above, there are problems in terms of the reduction in the size of the DC/DC converter and the reduction in the cost thereof.
According to a semiconductor device disclosed in Japanese Patent Application Laid-Open No. Sho 59-47753, an FET containing GaAs semiconductor and a Si diode for protection are packaged in a small package. However, the FET containing GaAs semiconductor and the Si diode can not be integrated on the same chip.
According to a composite semiconductor device disclosed in Japanese Patent Application Laid-Open No. Hei 1-144663, a power MOSFET and a diode are merely in the same container to make one package.
In Japanese Patent Application Laid-Open No. Hei 5-29908, there is disclosed only a minimized semiconductor relay, which has a photocoupler, a FET element, and four external terminals.
According to a DC/DC converter in Japanese Patent Application Laid-Open No. Hei 10-146047, a semiconductor device is provided on a wiring substrate. In the semiconductor device, a first switching means for controlling ON/OFF of a DC input power source, a second switching means for turning OFF/ON in synchronous with the first switching means (MOSFET including an internal diode and a Schottky barrier diode connected to the internal diode in parallel), and a synchronous controlling means are merely packaged in one body.
Furthermore, there is disclosed an FET rectifying circuit (Japanese Utility Model Application Laid-Open No. Sho 63-58898). The Application relates to a rectifying circuit in which an AC voltage for control is applied to the point between the gate and the source of an FET, and an AC voltage for rectification is rectified through a source-drain circuit of the FET. The FET rectifying circuit is provided with a Zener diode and a resistance. The Zener diode is connected to the gate-source circuit in series so that the polarity of the Zener diode is one in which the AC voltage for control of polarity on the conductive side of the FET is prevented. The resistance is provided in parallel the gate-source circuit.
However, these semiconductor devices can not improve the efficiency of the power source and the like.